Heat packs as described in the prior art have been long used in various forms in the medical and sports fields. Such heat packs were in the form of hot water bottles or supercooled salt hydrate solutions in flexible containers. Typical of the salt hydrate heat packs are those containing supercooled sodium acetate solutions which become exothermic when crystallization commences.
However, in order to initiate crystallization, the supercooled solution must first be seeded with seed crystal or other solid material capable of inducing crystallization.
The prior art is replete with devices designed to induce crystallization, but all of them have been less than totally acceptable, some being costly to manufacture and others being ineffective.
One way of inducing crystal formation as disclosed in the prior art has been to allow a small amount of supercooled solution to contact the air, permitting evaporation to take place which results in crystal formation. Although means for providing such induction of crystal growth would appear to be simple, the prior art has not disclosed any means for readily utilizing such method.
Other prior art means as disclosed in U.S. Pat. Nos. 549,925, 1,433,010, 1,481,208, 1,812,243, 1,887,613, 2,114,396, 2,220,777, 3,951,127, 4,077,390 and European Patent Application No. 0 045 971 are extremely varied but all require techniques that in the past have proved to be less than totally satisfactory.
U.S. Pat. No. 549,959 discloses a heat retaining device which employs sodium acetate in a vessel jacket. Crystallization is induced by unscrewing a stopper having a rod or needle attached and exposing it to air allowing crystallization to take place on the rod or needle which is then re-inserted into the vessel.
U.S. Pat. No. 1,433,010 discloses a device for storing heat in a shallow, cylindrical vessel containing a supersaturated solution of sodium acetate or another chemical which evolves heat upon crystallization. Crystallization is triggered by removing a screw cap to which is attached a rod inserted in the supersaturated solution. Upon withdrawal of the rod and wiping the rod surface dry, crystallization of sodium acetate residue on the rod is initiated. Upon reinsertion of the rod into the supercooled solution, crystallization is initiated. If crystallization of the solution does not occur, the rod is again withdrawn and seed crystals are introduced.
U.S. Pat. No. 2,220,777 discloses the use of an internal metal trigger which induces crystallization of a sodium acetate solution maintained in a sealed, valveless container.
U.S. Pat. No. 4,077,390 and European Pat. No. WO82/00417 both disclose a sealed heat pack containing an aqueous sodium acetate solution wherein crystallization is induced by the flexing or bending of an internal activator strip.
Other typical prior art devices used to induce crystallization of sodium acetate solutions are referred to in U.S. Pat. No. 4,077,390.
Accordingly, it is an object of the present invention to provide a simple and inexpensive re-usable means of inducing crystallization in supercooled solutions used in heat packs.